532 EXTERNAL ANATOMY OF INSECTS. 
ever examines the perfect insects of every Order, except 
the Aptera*, will find this distinction strongly pointed 
out, not only by the different direction of the first pair of 
legs from that of the two last, but also in a large pro- 
portion by a deep incisure; and in all it is further mani- — 
fested by the anterior segment having a motion distinct 
from that of the rest of the trunk, and separating readily 
from it; and this not only where it is large, as in insects 
that have a thoracic shield, but also in those in which 
the prothorax is less apparent: whereas the other two 
pedigerous segments have little or no distinct motion, 
will not readily separate from each other, and in some 
cases exhibit no pectoral suture between them. Some- 
times, however, these two last segments are more promi- 
nently distinguished: in Cantharis, Mylabris, and other 
vesicatory beetles, they are separated below by an inci- 
sure, or rather the first or mid-leg segment, is not nearly 
so elevated as that of the hind-legs. In some ants ((co- 
doma), in the neuters, there is no distinction of segments 
in the trunk; but in others (Formica) it follows the 
general law, and consists of three. In the Arachnida, 
with the exception of Galeodes, in which the head is di- 
stinct, and the three segments of the trunk may be traced, 
these parts together form only a single segment. In- 
duced by these reasons, I consider the trunk as consist- 
ing in general of two primary segments, the manitrunk 
and alitrunk: the latter resolvable into two secondary 
ones. 
* Manitruncus®,—The manitrunk, then, is the ante- 
4 In Nirmus Anseris, &c. however, in this Order, the same distinc- 
tion is observable. 
> Prates VIII. & IX. a. 
a 
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